The present invention relates to a photosensitive membrane, particularly to a photosensitive member comprising a hydrogen-containing carbon layer as a charge transporting layer.
The technique of an electrophotography has been remarkably developed since the invention of the image-transfer type, and also various new materials have been developed and have been practised.
The main materials for conventional electrophotosensitive members include inorganic compounds such as non-crystalline selenium, selenium-arsenic, selenium-tellurium, zinc oxide, amorphous silicon and the like, and organic compounds such as polyvinylcarbazole, metal phthalocyanine, dis-azo pigments, tris-azo pigments, perillene pigments, triphenylmethanes, triphenylamines, hydrazones, styryl compounds, pyrazolines, oxazoles, oxadiazoles and the like.
Structures of electrophotosensitive members include a single layer type using one of the above compounds, a binder type in which the above compounds are dispersed in a binder resin, and multilayer type carrier generating layers and carrier transporting layers.
All conventional materials for electrophotosensitive members, however, have defects, one of which is poisonous for humans. Additionally, in order to use these electrophotosensitive members in a copying machine, the initial properties must be kept constant when they are exposed to the serious copying conditions of charge, exposure, development, transference, erasing, cleaning and the like. Many organic compounds are poor in durability, and have many unstable properties.
Recently, in order to improve the above problems amorphous silicon (referred to as a--Si hereinafter) formed by a plasma chemical vapor deposition (referred to as plasma CVD) has been applied to produce a photosensitive member.
A--Si photosensitive members have several excellent properties. But the relative dielectric constant (.epsilon.) of a--Si is so large (about 12) that it essentially needs a thickness of at least 25 .mu.m to gain a sufficient surface potential for a photosensitive member. In addition, in the production of an a--Si photosensitive member by plasma CVD a long production time is needed because of a slow deposition rate of an a--Si layer, and the long deposition time makes it difficult to obtain a homogeneous a--Si layer with the result that image defects such as white spot noises are liable to occur at a high percentage. Further, the cost becomes expensive.
Though many attempts to improve the above defects have been made, it is not preferred to make the layers thinner.
On the other hand, an a--Si photosensitive member has additional defects such as weak adhesive strength between the a--Si layer and electroconductive substrate, and poor resistances to corona, external circumstances and chemicals.
It has been proposed to use an organic polymeric layer produced by plasma polymerization (referred to as OPP layer hereinafter) which is arranged as an overcoat layer or an undercoat layer in order to solve the above problems. The former is proposed, for instance, in U.S. Pat. No. 3,956,525, and the latter is done in Japanese Patent Kokai No. 63541/1985.
It is known that an OPP layer can be produced from various kinds of organic compound such as ethylene gas, benzenes, aromatic silanes and the like (e.g. Journal of Applied Polymer Science Vol. 17, 885-892 (1973), by A. T. Bell et al.). However, the OPP layer produced by these conventional methods is restrictively used as an insulator. Therefore, the layer is considered as an insulating layer having an electrical resistance of about 10.sup.16 .OMEGA..cm such as an ordinary polyethylene layer or at least similar to such a layer.
Recently, there is proposed a layer comprising diamond-like carbon in the semiconductor field. But charge transportability thereof has not been suggested at all.
U.S. Pat. No. 3,956,525 discloses a photosensitive member consisting of a substrate, a sensitizing layer, an organic photoconductive electrical insulator and a glow discharging polymer layer having a thickness of 0.1-1 .mu.m in the above order. This polymer layer is provided to cover the surface so as to stand up to wet development as an overcoat. Carrier transportability of the layer is not suggested.
Japanese Patent Kokai No. 63541/1980 discloses a photosensitive member comprising an undercoat layer composed of a diamond-like carbon and having a thickness of 200 .ANG. to 2 micron and an a--Si photoconductive layer formed on said undercoat layer. This undercoat layer is formed to improve adhesion of the a--Si layer to the substrate. The undercoat layer may be so thin that a charge moves through it by tunnel effect.
As mentioned above, photosensitive members have been proposed which comprises an undercoat layer composed of an electrically insulating OPP layer, a diamond-like layer and the like, but charge transportability is basically attributed to the tunnel effect and the phenomena of dielectric breakdown.
The tunnel effect is caused due to the pass of electrons, when the thickness of an insulating layer is very thin (generally at an Angstrom unit).
The dielectric breakdown phenomenon is where existing small amounts of charge carriers are accelerated by an electric field to gain a sufficient energy capable of ionizing atoms in the insulator, with the result that carriers increase by the ionization. This phenomena occurs at a high electric field (generally more than 100 V/.mu.m).
In the case of a photosensitive member having laminated layers of an insulating layer and a semiconducting layer, charges generated in the semiconducting layer move through the layer under an electric field, but they can not pass through the insulating layer under a low electrical field. If the insulating layer is thin, it is ignored as a surface potential or it does not affect adversely properties of photosensitivity because of negligible development influence. Further, even if the charges are accumulated on the insulating layer by repeated use to give a higher potential, the potential in the electric field does not increase above a constant level (e.g. 100 V/.mu.m) because of the dielectric breakdown.
For example, when an insulating layer comprising insulating materials capable of causing dielectric breakdown at 100 V/.mu.m is formed at a thickness of 0.1 .mu.m, the increase of the residual potential based on the repetition is only 10 V.
According to the above reasons, it has been understood that if a conventional insulating layer is used in a photosensitive member, the thickness of the layer has to be less than about 5 .mu.m, or else the residual potential based on the insulating layer increases to more than 500 V, so that an overlap of copied image occurs.